On Fri, 6 Sep 2024 10:28:11 -0400, Alan Stern wrote:
> On Fri, Sep 06, 2024 at 10:11:03PM +0800, Edward Adam Davis wrote:
> > The syzbot reported a kernel-usb-infoleak in usbtmc_write.
> >
> > The expression "aligned = (transfersize + (USBTMC_HEADER_SIZE + 3)) & ~3;"
> > in usbtmcw_write() follows the following pattern:
> >
> > aligned = (1 + 12 + 3) & ~3 = 16 // 3 bytes have not been initialized
> > aligned = (2 + 12 + 3) & ~3 = 16 // 2 bytes have not been initialized
> > aligned = (3 + 12 + 3) & ~3 = 16 // 1 byte has not been initialized
> > aligned = (4 + 12 + 3) & ~3 = 16 // All bytes have been initialized
> > aligned = (5 + 12 + 3) & ~3 = 20 // 3 bytes have not been initialized
> > aligned = (6 + 12 + 3) & ~3 = 20 // 2 bytes have not been initialized
> > aligned = (7 + 12 + 3) & ~3 = 20 // 1 byte has not been initialized
> > aligned = (8 + 12 + 3) & ~3 = 20 // All bytes have been initialized
> > aligned = (9 + 12 + 3) & ~3 = 24
> > ...
>
> What is the purpose of aligned? Why doesn't the driver simply use
> USBTMC_HEADER_SIZE + transfersize instead of rounding it up to a
> multiple of 4?
I just found out that the logic of aligned calculation is like this.
As for why it is calculated like this, perhaps Guido Kiener can provide
a clearer explanation.
It was introduced by commit 4d5e18d9ed93 ("usb: usbtmc: Optimize usbtmc_write").
>
> > Note: #define USBTMC_HEADER_SIZE 12
> >
> > This results in the buffer[USBTMC_SEAD_SIZE+transfersize] and its
> > subsequent memory not being initialized.
> >
> > The condition aligned < buflen is used to avoid out of bounds access to
> > the buffer[USBTMC_HEADER_SIZE + transfersize] when "transfersize =
> > buflen - USBTMC_HEADER_SIZE".
> >
> > Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
> > Reported-and-tested-by: syzbot+9d34f80f841e948c3fdb@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
> > Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> > ---
> > drivers/usb/class/usbtmc.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/usb/class/usbtmc.c b/drivers/usb/class/usbtmc.c
> > index 6bd9fe565385..faf8c5508997 100644
> > --- a/drivers/usb/class/usbtmc.c
> > +++ b/drivers/usb/class/usbtmc.c
> > @@ -1591,6 +1591,10 @@ static ssize_t usbtmc_write(struct file *filp, const char __user *buf,
> > goto exit;
> > }
> >
> > + if (aligned < buflen && (transfersize % 4))
>
> Shouldn't this be
>
> if (USBTMC_HEADER_SIZE + transfersize < aligned)
Logically, it seems possible to write it this way.
>
> ?
>
> Alan Stern
>
> > + memset(&buffer[USBTMC_HEADER_SIZE + transfersize], 0,
> > + aligned - USBTMC_HEADER_SIZE - transfersize);
> > +
> > dev_dbg(&data->intf->dev, "%s(size:%u align:%u)\n", __func__,
> > (unsigned int)transfersize, (unsigned int)aligned);
BR,
Edward
